By Amy Reines and Andrea Comastri

Amy Reines and Andrea Comastri present a review paper about accretion in two extreme regimes: Super Massive Black Hole (SMBH, MBH ≥ 10⁸-10⁹ M_☉) demography at high and very high redshift and the smallest nuclear BH (MBH ≤ 10⁵M_☉) in present-day dwarf galaxies.

Athena deep and wide surveys will allow to detect X-ray emission, the smoking gun from the accreting SMBH from a large number of objects at z>6 (when the Universe had less than 7% of its present age) allowing a breakthrough in our understanding of BH formation and early evolution.

The figure shows the current situation where almost 80 objects at z > 6 are known mainly from SDSS and other optical near-ir surveys. Only a tiny fraction (~10% - the blue symbols) is detected in X-rays with current observatories.

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